This application relates generally to digital data storage devices and more particularly to a vibration damper for an actuator assembly in a disc drive.
The positioning accuracy of an actuator in a head disc assembly (HDA) of a disc drive must increase in order to achieve the increased aerial densities required of newer generations of disc drives. This accuracy depends substantially on the servo bandwidth and is limited by the inherent mechanical resonances of the actuator body or xe2x80x9cE-blockxe2x80x9d, the head suspension assembly (HSA) and disc pack vibration modes. There has been considerable progress in designing robust control systems that minimize the sensitivity to excitation forces. However, the systems cannot compensate completely for all mechanical resonances. Some of the vibration energy, due to such sources as disc pack rocking and translation modes, spindle bearing modes, and disc flow induced vibration modes, is transmitted through the disc drive motor spindle mounting screw fastened to the top cover of the HDA. These vibrations are in turn transmitted to the actuator through the top cover screw fastening the top cover to the actuator assembly. During installation, any misalignment of the top cover and actuator shaft holes induces transverse loads that increase the translational mode of the actuator.
In general, a higher servo bandwidth frequency provides greater immunity to all disturbances which cause head to track misregistration. The main reason for not going to very high bandwidths, however, is that actuator assembly resonances with a high gain in the off-track direction can cause servo instability. If the bandwidth is close to a resonant frequency with high gain, ringing will occur. This is very detrimental. Specifically, if the gain curve of an open loop bode plot exceeds zero decibels after the gain crossover frequency and before the phase curve is below xe2x88x9290 degrees, then unstable servo oscillations will occur. Typically, for every octave above the gain crossover, the resonant peak can be 3-4 db above the baseline response before 0 db will be crossed. In other words, the typical gain curve drops 3-4 db/octave after the gain crossover. In typical disc drive actuators the first resonance, which limits the bandwidth because of the modal gain, is the actuator translational mode. Attempts made to increase the frequency of this mode are usually met with very limited success. Accordingly, there is a need for a disc drive that has a damped translational mode where the transmission of these vibrations to the actuator assembly is minimized.
The hub cap actuator damper in accordance with the present intention damps actuator vibrations and improves the head on track performance. Lowering the gain of the mode so that the bandwidth can be placed close to the mode without the peak crossing zero db does this. The hub cap damper is an annular vibration absorbing disc placed between the cover and the actuator bearing cartridge which dissipates the energy in the visco-elastic layer and provides acoustic isolation between the disc motor spindle and the actuator assembly. Further, this isolation also improves the operational shock level the disc drives can withstand due to the actuator and spindle coupling. The deformations which take place in the translational mode put virtually all the reaction force on the actuator shaft which is in turn restrained by the base and the top cover. All of the materials that share in the modal strain of the translational mode have very low loss factors. The addition of an annular vibration-absorbing disc placed between the cover and the actuator bearing cartridge adds a high loss factor material in an area that has significant modal strain thus reducing the gain or amplitude of the resonance.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.